This invention relates to magnetic recording systems, and, more particularly, to a muting device for use when starting a recording operation.
The conventional use of magnetic recording systems in, for example, cine cameras includes a motion picture film provided with a magnetic coating in a stripe, so that the sound can be recorded at the same time as when motion pictures are taken, and the so-called simultaneous sound recording cine cameras are put into practical use.
With such cameras, when taking a motion picture with sound accompaniment, and particularly at the time when the film starts to move, it takes a certain length of time for the film transport system comprising the capstan and the fly wheel to attain stabilization.
Therefore, making a sound recording during this time is not recommended because, upon later reproduction, the sound pitch is distorted. Nevertheless, since particularly in sound motion picture cameras the time necessary to take one scene is very short, much account is given the recording and reproducing characteristics at the time of start of each scene. Further, in many cases, the motion pictures and the sound accompaniment are recorded on one and the same medium, so that it is very difficult to cut off that portion of the recording medium which reproduces noise.
To eliminate such drawbacks, according to the prior art, devices as shown in FIGS. 1 and 2 have been proposed. The device of FIG. 1 is applied to a sound recording system in the simultaneous sound recording cine camera. The system is shown in block form as comprising a signal input 1, for example, in the form of a jack for microphone, a pre-amplifier 2, and a sound volume adjusting means 3. A muting circuit 4 includes a resistor R.sub.1 and a transistor Q.sub.1 as a variable impedance element. The system further includes an amplifier 5 for sound recording, a second amplifier 6 for monitor with an output terminal 7 in the form of a jack for monitor, a constant current forming and equalizer circuit 8 which comprises a resistor R.sub.2 having a large resistance value as compared with the impedance of a magnetic head 11 to be described later, and a condenser C.sub.1. A superimposing circuit 9 for superimposing the input signal on a bias from a bias oscillator 10 comprises a condenser C.sub.2 resonant with the frequency of the bias, a resonator circuit having an inductor L.sub.1, and a condenser C.sub.3. Also shown is the bias oscillator 10 for high-frequency bias, and the magnetic head 11. A switch 12 is closed when a camera release button (not shown) is actuated. A mono-stable multivibrator 13 produces a control signal of high level for a predetermined time after the closure of said switch 12 at an output terminal thereof, the output terminal being connected to the base of the transistor Q.sub.1.
In operating the system of such construction, prior to the taking of a motion picture, a main switch (not shown) is thrown to supply the sound recording system of FIG. 1 and an exposure control system (not shown) with electrical current, thereby the pre-monitor of a sound recording, and picture taking, are made possible. That is, a more detailed explanation about the sound recording system is given below by reference to FIG. 1. The jack 1 is supplied with a sound signal from a microphone (not shown). Then, the sound signal is amplified by the pre-amplifier 2 and, after having been adjusted to a proper level by the control means 3, is then supplied to the muting circuit 4. Here the potential at the base of the transistor Q.sub.1 is yet of low level, permitting the sound signal to be applied without attenuation to the amplifier 5 for sound recording which then produces two outputs. One of the outputs is applied to the amplifier 6 for monitor and therefrom to the jack 7 for monitor. With the use of a monitor device such as an ear-phone, as connected to the jack 7, the sound which is about to be recorded can be monitored. The other output of the amplifier 5 for sound recording is applied to the constant current forming and equalizer circuit 8 and is then superimposed on the bias from the oscillator 10 before it is applied to the magnetic head 11. Since in this stage the film is not yet run, a sound recording is not initiated. Then, when the camera release button is actuated to initiate a picture taking operation, the film is caused by a mechanism (not shown) to move. At this time, the switch 12 is synchronously closed, causing the mono-stable multi-vibrator 13 to produce a signal which remains at high level until the film transport system including the capstan and fly wheel (not shown) becomes stable from the output terminal thereof. Responsive to this signal, the transistor Q.sub.1 lowers the impedance between the collector and emitter thereof sufficiently as compared with the resistance value R.sub.1. Therefore, the output signal of the amplifier 3 is barely transmitted to the amplifier 5. In short, so long as the output of the multi-vibrator 13 is of high level, the monitor jack 7 and magnetic head 11 are not supplied with a sound signal. After that, as the film transport system becomes stable, the output of the mono-stable multi-vibrator 13 changes to low level, so that the impedance between the collector and emitter of the transistor Q.sub.1 becomes high as compared with the resistor 11, and there is little attenuation by the muting circuit 4. Therefore, a sound signal flow to the monitor jack 7 and magnetic head 11 is initiated, thus enabling the sound signal to be monitored and to be recorded. In other words, the actuation of the muting circuit is made to continue until the film transport system becomes stable and, therefore, no sound recording is made possible during that time, thus making it possible to prevent the reproduction of a sound of different pitch during film reproduction. But, such arrangement has a drawback that after the actuation of the camera release, no monitor sound can be heard so long as the muting circuit is under operation. Another drawback is that since during the pre-monitor before the actuation of the camera release a portion of the sound signal is supplied to the magnetic head, the magnetic band is, though linewise, magnetized through the magnetic head. This becomes a cause of producing noise when the film is being reproduced. To eliminate these drawbacks, an alternate arrangement of the muting device as shown in FIG. 2 has been considered. The same reference numerals have been employed in FIG. 2 to denote similar parts to those shown in FIG. 1, and no more explanation of their operation is given below.
In the arrangement of FIG. 2, the muting circuit 4a is connected to the output side of the constant current forming and equalizer circuit 8. Further, instead of the mono-stable multi-vibrator 13, there is provided a muting signal forming circuit 13'. The muting signal forming circuit 13' comprises a mono-stable multi-vibrator 13, a logic OR circuit OR.sub.1 and a resistor R.sub.3, these parts being arranged as shown in FIG. 2.
Therefore, said muting signal forming circuit 13' produces an output signal which goes to high level when in the pre-monitor prior to the actuation of release, or when the film running after the release is unstable, and which changes to low level when the film running is stabilized after the release.
With this construction and arrangement, during the time when muting is in operation, there is no possibility of loss of monitor sound, and since during the monitor no sound signal is supplied to the magnetic head, there is also no possibility of occurrence of magnetization of the sound recording track on the film. Thus, the drawbacks of the system of FIG. 1 are eliminated. However, unlike said muting circuit which is so constructed that attenuation is carried out by the divided voltage with the impedance of the constant current forming resistor R.sub.2 and the saturated resistance value of the transistors Q.sub.1 ', in many cases a condenser C.sub.1 is connected to the constant current forming resistor R.sub.2 in parallel to result in an increase of the high-pass sound recording current value, thereby forming an equilizer for sound recording.
For this reason, when the muting transistor Q.sub.1 ' is saturated, the amount of attenuation at the high frequency is not very sufficient since it depends upon the impedance of the condenser C.sub.1 and the impedance of the transistor Q.sub.1 ' when saturated.
Another problem arises from the fact that the magnetic head adapted to be used in the simultaneous sound cine camera is difficult to increase in bulk and size from the standpoint of compatibility with the film magazine, and, therefore, the number of turns of the coil around the head core cannot be set at a desired high value, thus leading to the impossibility of providing for a head with a large impedance. Particularly at low frequencies, the impedance is at a low value.
Therefore, when muting, the impedance cannot assume a sufficiently high value as compared with the saturation resistance value of the transistor Q.sub.1 ', and this problem becomes serious particularly when the frequency is low. Thus, it is difficult to assure a sufficient muting effect not only in the high region, but also in the low region.